Proton conductivity of the polymer electrolyte membrane (PEM) is one of the key factors limiting the performance of polymer electrolyte membrane fuel cells (PEMFCs). Significant enhancement of proton conductivity in perfluoro sulfonic acid based membranes that are commonly used today in PEMFCs is not only an urgent need, but also a highly valuable contribution to the field.
There are several attempts in the prior art to improve proton conductivity of polymer electrolyte membranes such as Zhai et al in Journal of Membrane Science 280 (2006) 148-155 has shown that proton conductivity can be increased by retaining water and Pereira et al in Chem. Mater. 2008, 20, 1710-1718 showed enhanced performance by incorporating meso porous silica in the Nafion matrix. However, there are no prior reports on the use of functionalized carbon nanostructures to enhance the proton conductivity of such membranes.
Further, there are several reports in the prior art of compositions using a polymer electrolyte and carbon nanostructures as in U.S. Pat. No. 7,361,430, which discloses a carbon nanotube (SWNT)-polymer composite actuator and method to make such actuator and WO/2006/016907 titled “Photo Actuators Based on Carbon Nanotube-Nafion Composites” discloses a photo-mechanical actuating material that is composed of an organic composite and a method of forming such a structure.
Nafion-carbon nanotube composites have been reported for fuel cell electrolyte applications by Liu et al in Electrochemical Solid-State Letters. 2006, 9, A356. These composites however showed no significant improvement in proton conductivity.
Thomassin et, al in Journal of membrane science 303, 2007, 252-257 conclude that nafion membranes modified with multi-walled carbon nanotubes with carboxyl groups significantly decreased the methanol permeability, but only slightly decreased ionic conductivity.
However, none of these prior art documents report a composition of a polymer electrolyte with suitably functionalized carbon nanostructures that can significantly enhance proton conductivity.
The objective of the invention is to have a novel composition of proton conducting polymers with carbon nanostructures which enhance proton transport. The prior art review clearly indicates that sulphonic acid membranes and carbon nanostructures, a combination of these, processes for preparing the membranes and their various applications are known. Method of functionalizing nanomaterials with various functions is disclosed in WO 2006/099392. Use of sulphonic acid membranes with carbon nanostructures in electrodes of polymer electrolyte fuel cells is known in literature. Further electron transport enhancement of said materials is also discussed in literature. However, when the inventors prepared a composition of sulphonic acid membranes with sulphonic acid functionalized carbon nanostructures, it was surprisingly found that there is at least a one log enhancement of proton transport.
Thus in this invention, we disclose novel compositions of proton conducting polymers with suitably functionalized carbon nanostructures that show a surprising enhancement in proton conductivity.